This invention relates generally to electromagnets. More particularly, it relates to methods for designing homogeneous field electromagnets for use in magnetic resonance imaging (MRI) or prepolarized magnetic resonance imaging (PMRI).
Magnetic resonance imaging (MRI) is a common and well known technique for imaging the internal structure of objects and for medical diagnosis. Conventional MRI requires that the object to be imaged be placed in a homogeneous (typically to within 40 ppm) and strong (typically in the range of 0.5 to 1.5 Tesla) magnetic field. Generating such magnetic fields is difficult and expensive.
Prepolarized MRI (PMRI) is a recent technique which uses a strong, nonhomogeneous pulsed magnetic field in combination with a weaker, homogeneous magnetic field to perform imaging. The strong, pulsed field is known as the polarizing field and it is produced by a polarizing magnet. The weaker, homogeneous field is known as the readout field and is produced by a readout magnet. PMRI is also referred to as switched-field MRI and is related to field cycling nuclear magnetic resonance (NMR) relaxometry.
In PMRI, the polarizing field is switched on briefly (about 0.01 to 2 seconds) to polarize the nuclear spins inside the object to be imaged. Then, the polarizing field is rapidly reduced at a rate faster than the decay rate of the nuclear spin polarization. The nuclear spin polarization is then analyzed in the readout magnetic field. The polarizing field causes the nuclear spin polarization to be greater than it would be with only the readout field. Reference can be made to U.S. Pat. No. 5,629,624 to Carlson et al., U.S. Pat. No. 4,906,931 to Sepponen, and U.S. Pat. No. 5,057,776 to Macovski concerning PMRI.
A result of the pulsed polarizing magnetic field is that it renders a large PMRI device very difficult to build. The magnetic energy stored in the magnet must be removed and restored with every pulse. This practically limits the amount of energy which can be stored in the pulsed magnet and thus the size of the PMRI device. Therefore, future PMRI devices will likely be small dedicated imagers, dedicated to imaging small body parts such as hands, feet, knees, heads, breasts, neck and the like.
Imaging small body parts places limitations on magnet geometry (both readout and polarizing). Most body parts are not cylindrical and therefore do not efficiently occupy the volume inside a traditional cylindrical magnet assembly. A cylindrical magnet assembly is a collection of coils arranged on the surface of a cylinder. Access to the magnetic field of a cylindrical magnet is limited to the end openings of the cylinder or between the coils. This limited access makes it difficult and uncomfortable to image certain body parts such as knees. It is particularly difficult to provide a magnet for imaging a knee or elbow as it is flexing.
It would be an advance in the art to provide readout magnet designs which allow increased access to the homogeneous magnetic field. Such improved magnet designs would be particularly well suited for use in dedicated PMRI machines.
Lee-Whiting discloses a design for a 4-coil biplanar magnet in xe2x80x9cHomogeneous Magnetic Fieldsxe2x80x9d, Atomic Energy Commission of Canada Limited CRT-673, 1-29 (1957). The magnet has 2 coils symmetrically and coaxially disposed in each of two parallel planes. The homogeneous magnetic field is located between the planes defined by the coils. The homogeneous magnetic field can be accessed from the radial direction (i.e., from between the planes defined by the coils). A similar 4-coil design is also disclosed by Garrett in xe2x80x9cThick Cylindrical Coil Systems with Field or Gradient Homogeneities of the 6th to 20th Orderxe2x80x9d, Journal of Applied Physics, 38, 2563-2586 (1967). A shortcoming of these 4-coil designs is that they are relatively inefficient in producing the desired homogeneous field, and produce relatively inhomogeneous magnetic fields.
U.S. Pat. No. 4,829,252 to Kaufman discloses an MRI system with improved patient access to the magnetic field. The system of Kaufmann uses biplanar magnets to produce the required homogeneous magnetic field. Kaufman does not disclose how to design the biplanar magnets or specific biplanar magnet designs.
Accordingly, it is a primary object of the present invention to provide a method for designing magnets that:
1) produces biplanar magnet designs with improved magnetic field access compared to prior art designs;
2) produces biplanar magnet designs with exceptional magnetic field homogeneity;
3) can be used to design biplanar magnets with relatively large number of coils;
4) can be used to design magnets having a variety of desired properties such as increased efficiency, certain size constraints, certain inductance values or reduced power consumption.
It is a further object of the present invention to provide biplanar magnets that:
1) produce exceptionally homogeneous magnetic fields;
2) are relatively efficient;
3) have improved access o the homogeneous magnetic field.
It is also an object of the present invention to provide an apparatus for prepolarized magnetic resonance imaging (PMRI) that:
1) has improved access to the imaging region;
2) can be adapted to image many different body parts.
These and other objects and advantages will be apparent upon reading the following description and accompanying drawings.
These objects and advantages are attained by a 6-coil biplanar symmetrical electromagnet. The 6-coil magnet has 3 coils symmetrically and coaxially disposed in each of two parallel planes. The coils have radii and are designed to carry accurately determined currents (in units of Ampere-turns). The 6-coil magnet provides an accurately homogeneous magnetic field between the two planes. The coils enclose an ideal filamentary current loop calculated according to a method of the present invention. The magnet may further include electronics for providing the accurately controlled currents for the coils. The present invention also includes 8-coil and 10-coil biplanar symmetrical electromagnets.
The magnet may also include a polarizing magnet for providing a polarizing magnetic field needed for performing prepolarized magnetic resonance imaging (PMRI). The polarizing magnet can be oriented in many different ways.
The present invention also includes a method for making biplanar symmetrical electromagnets having K coils. The method begins with producing equations for the spherical harmonic coefficients describing the magnetic fields produced by the K coils. The equations for the spherical harmonic coefficients are set equal to zero, which then allows a numerical optimization program to solve the equations for the coil radii, coil locations, and coil currents in units of Ampere-turns. The number of coils K can be equal to or greater than 6.
The present invention also includes magnets made according to the method of the present invention.
Also, the present invention includes an apparatus for performing prepolarized magnetic resonance imaging, (PMRI). The apparatus has a biplanar, symmetrical readout electromagnet made according to the present invention and a polarizing magnet. The readout electromagnet provides a homogeneous magnetic field. The polarizing magnet can have many different orientations with respect to the readout electromagnet.
The present invention includes an alternative method for making electromagnets that uses a reduced number (i.e., less than Kxe2x88x921) of spherical harmonic constraints. Preferably, other nonspherical-harmonic constraints are used in combination with the spherical harmonic constraints. The nonspherical-harmonic constraints can be based on magnetic field efficiency, magnet size, magnet aspect ratio, power consumption or coil conductor volume. The alternative method produces slightly altered magnet designs with somewhat reduced field homogeneity, but with enhancement of other characteristics.